US 6337646 B1 Abstract To provide a D/A converter and a D/A converting method in which a nonlinear error of an analog output obtained in accordance with a digital input can be decreased without using any specific analog process.
An n-bit D/A converter (
2) includes: correction signal generating means (4) for generating an m-bit digital correction signal (wherein m is a positive integer) in accordance with an n-bit digital input signal D (wherein n is a positive integer of 2 or more); and D/A conversion means (6) for converting an (n+m)-bit digital signal consisting of the n-bit input signal D and the m-bit correction signal into an analog signal.Claims(15) 1. A digital-to-analog converter comprising:
non-programmable correction signal generating means for generating an m-bit digital correction signal, wherein m is a positive integer, in accordance with an n-bit digital input signal, wherein n is a positive integer of 2 or more; and
digital-to-analog conversion means for converting an (n+m)-bit digital signal consisting of said n-bit input signal and said m-bit correction signal into an analog signal.
2. The digital-to-analog converter according to
3. The digital-to-analog converter according to
4. The digital-to-analog converter according to
5. The digital-to-analog converter according to
6. The digital-to-analog converter according to
7. The digital-to-analog converter of
8. The digital-to-analog converter of
9. The digital-to-analog converter of
10. A digital-to-analog converting method comprising:
a correction signal generating step of generating an m-bit digital correction signal, wherein m is a positive integer, in accordance with an n-bit digital input signal, wherein n is a positive integer of 2 or more, said correction signal being generated from a non-programmable digital circuit; and
an (n+m)-bit digital-to-analog converting step of converting an (n+m)-bit digital signal consisting of said n-bit digital input signal and said m-bit digital correction signal into an analog signal.
11. The digital-to-analog converting method according to
12. The digital-to-analog converting method according to
13. The digital-to-analog converting method according to
14. The digital-to-analog converting method according to
15. The digital-to-analog converting method according to
Description The present invention relates to a digital-to-analog converter and a digital-to-analog converting method, and more particularly, it relates to a digital-to-analog converter and a digital-to-analog converting method in which a nonlinear error of an analog output resulting from digital-to-analog conversion (D/A conversion) is decreased without using any specific analog process. With recent high integration of large scale integrated circuits (herein after referred to as LSIs), there are increasing demands for LSIs mounting both analog circuits and digital circuits. A high-accuracy and low-cost digital-to-analog converter (hereinafter referred to as a D/A converter) and a high-accuracy and low-cost analog-to-digital converter (hereinafter referred to as an A/D converter) are required to be mounted on such LSIs. Therefore, it is significant to realize a high-accuracy analog circuit by using a general semiconductor process without using a high-accuracy but high-priced analog process. As an example of a D/A converter, an n-bit D/A converter The switches Sn, Sn- Since this R-2R resistance net However, when resistors have voltage dependence, problems will arise even if the resistors of uniform properties are used. For example, in the case where the R-2R resistance net is formed by using a semiconductor process in which no high-accuracy resistance element is particularly used, the resistance element is often formed in an n-type diffused layer. However, depending on a voltage applied to the resistors, a depletion layer formed between the resistor and a p-type substrate is changed, so that the resistance value can be varied. When the resistance value is thus varied by the voltage applied to the resistor, it is difficult to achieve a high-accuracy D/A converter. For example, FIG. 8 is a simplified circuit diagram of a 2-bit D/A converter (including a ladder circuit)
As shown in TABLE 1, since the resistors r FIG. 9 shows a relationship between an error ΔV (=Vout−Vr) generated between an analog output voltage Vout and an ideal voltage Vr and a digital input signal D (which is shown in a range from 0 (zero) to FS (full scale)) in a 10-bit D/A converter using such voltage-dependent resistors in a ladder circuit. The ideal voltage Vr is increased by a predetermined voltage with an increase of a least significant bit (LSB) of the input signal D, and a relationship between the input signal D and the ideal output voltage Vr is substantially linearly varied. In FIG. 9, assuming that the error is substantially zero when the input signal D is 0 and FS, the error ΔV from the ideal output line (nonlinear error) reaches to the maximum error ΔV_max in the vicinity of ½FS. This maximum error ΔV_max largely affects the conversion accuracy of the D/A converter. Thus, there is a need for a high-accuracy D/A converter and a high-accuracy D/A converting method in which a nonlinear error caused by a voltage dependence of a resistance element that degrades the accuracy of a D/A conversion can be corrected without using any specific analog process. In the D/A converter of the present invention, an error (nonlinear error) between the output voltage of the D/A converter and an ideal output voltage can be corrected by using a correction signal generated by a correction circuit. By using the D/A converting method of the present invention capable of correcting the output voltage by using the correction signal, a high-accuracy and low-cost D/A converter can be realized without using any specific analog process. The D/A converter of the present invention is characterized by comprising correction signal generating means for generating an m-bit digital correction signal (wherein m is a positive integer) in accordance with an n-bit digital input signal (wherein n is a positive integer of 2 or more); and digital-to-analog conversion means for converting an (n+m)-bit digital signal consisting of the n-bit input signal and the m-bit correction signal into an analog signal. The D/A converting method of the present invention is characterized by comprising: a correction signal generating step of generating an m-bit digital correction signal (wherein m is a positive integer) in accordance with an n-bit digital input signal (wherein n is a positive integer of 2 or more); and an (n+m)-bit digital-to-analog converting step of converting an (n+m)-bit digital signal consisting of the n-bit digital input signal and the m-bit digital correction signal into an analog signal. FIG. 1 is a block diagram showing the basic structure of a D/A converter according to the present invention. FIG. 2 is a block diagram showing an embodiment of the D/A converter of the present invention. FIG. 3 is a circuit diagram showing an embodiment of a correction circuit in the D/A converter of FIG. FIG. 4 is a graph roughly illustrating an error between the output voltage of the D/A converter of FIG. FIG. 5 is a circuit diagram showing another embodiment of the correction circuit in the D/A converter of FIG. FIG. 6 is a graph roughly illustrating an error between the output voltage of the D/A converter using the correction circuit of FIG. FIG. 7 is a circuit diagram showing an embodiment of a conventional n-bit D/A converter. FIG. 8 is a simplified circuit diagram of the n-bit D/A converter of FIG. 7 wherein n=2. FIG. 9 is a graph roughly illustrating an error between the output voltage of the n-bit D/A converter of FIG. 7 wherein n=10 and an ideal output voltage. Referring now to the accompanying drawings, preferred embodiments of a D/A converter according to the present invention will be described in detail below. FIG. 1 is a diagram showing the basic structure of the D/A converter of the present invention. The n-bit D/A converter FIG. 2 is a diagram showing an application of the present invention to a D/A converter for a 10-bit signal. In FIG. 2, the D/A converter The correction circuit
The 12-bit D/A converter Similarly, when the correction signal “D In this manner, as shown in TABLE 2 and FIG. 4, in the ranges between 0 and ⅛·FS (i.e., when “D As a result, as shown with a dashed line Having described an embodiment of the present invention, the D/A converter and the D/A converting method of the present invention can also be materialized in other embodiments. For example, when a 7-bit correction signal is used, the following correction can be carried out in a maximum signal (i.e., “1111111”), with the least significant bit of an input signal indicated as LSB:
In this manner, the correction corresponding to the LSB at maximum can be carried out by increasing the number of bits of the correction signal. However, when the number of bits of the correction signal is increased, the number of components of the D/A converter (such as resistors R and 2R, switches S, and the like) is increased in accordance with the increase of the number of bits. Therefore, the number of bits of the correction signal is preferably as small as possible in consideration of compactness and low cost of the D/A converter. Moreover, the digital correction signal generated by the correction circuit in accordance with the input signal is not limited to the outputs as shown in TABLE 2, but an arbitrary digital signal can be generated in accordance with the output characteristic of the D/A converter. Specifically, a correction signal with respect to respective bits of an input digital signal can be set on the basis of the nonlinear error of the input signal. In general, the correction signal is maximized when the nonlinear error is maximum with respect to the input signal and it is minimized when the nonlinear error is minimum with respect to the input signal. For example, in the case where the D/A converter has an output characteristic
Having described the embodiments of the D/A converter and the D/A converting method according to the present invention with reference to the accompanying drawings, the present invention is not limited to the illustrated converters and the converting methods. It will be apparent to those skilled in the art that verious changes, modifications, and improvements can be made thereto without departing from the spirit or scope of the present invention. Patent Citations
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